Compression spring ends affect variables in the overall
design; solid height, active coils, free length, and the way the spring
seats in a given application. Below are illustrations of the four
basic ends available in compression springs.

A note on springs ends, taken from the 'Handbook of
Spring Design' by SMI:

"A compression spring cannot be closed and ground so consistently
that its ends will always be square (in parallel planes at right angles
to its axis). In addition, the helix angles adjacent to the end
coils will not have uniform configuration and closing tension, and these
springs cannot be coiled so accurately as to permit all coils to close
out simultaneously under load. As a result of these end coil
effects, the spring rate tends to lag over the initial 20 percent of the
deflection range, often being considerably less than calculated.
As the ends seat during the first stage of deflection, the spring rate
rises to the calculated value. In contrast, the spring rate for
the final 20 percent of the deflection range tends to increase as coils
progressively close out.

"The spring rate over the central 60 percent of the
deflection range is essentially linear. If possible, critical
loads and rates should be specified within this range. Which can
be increased to about 80 percent of total deflection by special
production techniques. However, these techniques add substantially
to manufacturing cost and are usually unwarranted."